1. Field of the Invention
The present invention relates to a method and a system for allocating bandwidths for use in a telecommunications network having available channels limitative, such as a wavelength division multiplex passive optical network (WDM-PON) and a code division multiplex access passive optical network (CDMA-PON).
2. Description of the Background Art
FIG. 1 is a schematic system diagram useful for understanding an exemplified allocation of bandwidths for communications between central-office (CO) equipment 10 and subscriber terminals, e.g. personal computers, through network devices for subscriber, i.e. subscriber units 11 to 14. In the system having bandwidths limitative which are available between the central-office equipment 10 and the subscriber units 11 to 14, as shown in FIG. 1, the subscriber units 11 to 14 are adapted to apply for a parameter, such as a bandwidth, for communication to the central-office equipment 10 (step ST1). If the application from the subscriber units 11 to 14 is acceptable to the central-office equipment 10, it is then accepted, and otherwise it is rejected (step ST2). The subscriber units 11 to 14 will use respective bandwidths accepted by the central-office equipment 10 for the communication. If communication is tried on a bandwidth other than those accepted, i.e. violates the rejection, information on that communication will be discarded (step ST3). In another solution, in order to prevent the discard of information due to the violation, a rejected subscriber unit again requests the central-office equipment 10 for transmission before starting the communication, and when the subscriber unit receives permission for transmission and is allotted transmission timing or the bandwidth of a channel from the equipment 10, it will use the received information to proceed to the communication, the procedure being repeated until successful.
FIG. 2 is a conceptual diagram useful for understanding an optical access network as an example of the prior art system shown in FIG. 1. As seen from the figure, this example of optical access network includes an OLT (Optical Line Terminal) 20 provided as central-office equipment, ONUs (Optical Network Units) 21 to 24 provided as subscriber units, an optical beam coupler/splitter 25, optical fibers 26a to 26d respectively connecting the ONUs 21 to 24 to the coupler/splitter 25, and another optical fiber 27 connecting the OLT 20 to the coupler/splitter 25. Each of the ONU has to transmit an up-going optical signal at the timing of causing no collision of the signal to be transmitted against another up-going signal transmitted from another ONU in the coupler/splitter 25. The transmission timing is reported from the OLT 20 to the ONUs 21 to 24.
FIG. 3 is a conceptual diagram useful for understanding a CDMA radio access system as another example of the prior art system shown in FIG. 1. In the radio communication system shown in FIG. 3, a base station as central-office equipment 30 and subscriber terminals 31 to 34 communicate with each other over air interface serving as a shared transmission medium with radio waves. In the CDMA system, a frequency bandwidth is divided with code to form several channels, e.g. CH1 to CH4, which are time-shared for communication. Such a solution of communication is also the same as an optical access system using the optical CDMA technique.
In the communications systems shown in FIGS. 1, 2 and 3, it is important how to allocate bandwidths to the respective subscriber units. As a solution useful for practicing the allocation, a two-stepped leaky bucket system is disclosed in, for example, Naoaki YAMANAKA, et al., “Performance Limitation of Leaky Bucket Algorithm for Usage Parameter Control and Bandwidth Allocation Methods” IEICE Trans. Commun., Vol. E75-B, No. 2, pp. 82-86, February, 1992. A traffic management system by UPC (Usage Parameter Control) in an ATM (Asynchronous Transfer Mode) network is disclosed in, for example, Naoaki YAMANAKA, et al., “ATM Network Traffic Management System by Deterministic UPC” IEICE Trans., Vol. J76-B-I, No. 3, pp. 253-263, March, 1993.
FIG. 4 is also a conceptual diagram useful for understanding the UPC in the leaky bucket system. For describing a bandwidth control in the leaky bucket system, a bandwidth control model is used, in which information as in the form of traffic, such as packet or token, is compared to “water” and a queue serving as means for holding information is compared to a “bucket with a hole”. As seen from FIG. 4, in the leaky bucket system, each input line uses two stepped “buckets with a hole” or queues 41 and 42 and a readout function therefrom. The “water” 43 to be inputted is supplied into the first step of “bucket with a hole” 41. This “water” to be inputted corresponds with information desired to be transmitted by the subscriber terminal. The amount of the information may increase or decrease, and possibly vanish. Out of the first step of “bucket with a hole” 41, a fixed amount of “water” flows per unit time, that is, a fixed amount of information per unit time is read out from the queue. A peak rate Tp applied for by the transmitting terminal corresponds to the size of the “hole” 41a of the first step of “bucket with a hole” 41, i.e. the amount of the information read out per unit time from the bucket 41. According to the volume 41b of the first step of “bucket with a hole” 41, an allowable amount of “water” violating, or exceeding, the peak rate is determined. If the input “water” violates the peak rate continuously for a long period, the “water” overflows from the first step of “bucket with a hole” 41 to be discarded.
According to the size of a “hole” 42a of the second step of “bucket with a hole” 42, the amount of information read out per unit time from the bucket 42 is determined, and the size corresponds to the size of an average bandwidth, i.e. average rate. According to the volume 42b of the second step of “bucket with a hole” 42, an allowable amount of “water” violating, or exceeding, an average flow rate Ta outputted from the “hole” 42a is determined. In other words, according to the volume of the second step of “bucket with a hole” 42, an allowable range of temporal bias of inflowing “water”, i.e. an arriving packet, is defined. Whenever the amount Tpe of information arriving in a certain period Pe exceeds Ta×Pe (i.e. Tpe>Ta×Pe), the violating traffic will overflow from the second step of “bucket with a hole” 42 to be discarded.
The mechanism shown in FIG. 4 can be installed in subscriber units (network devices) configured so as to input a packet (traffic) as “water”. However, as described in YAMANAKA, et al., the mechanism shown in FIG. 4 may be provided in the central-office equipment for all subscribers and the subscriber units are adapted to be responsive to control data sent from the central-office equipment to control the timing and period for transmitting user data. In that case, the “bucket with a hole” receives an application for a communication bandwidth from a subscriber unit and a token as traffic information in the queue installed in the subscriber terminal, and sends a transmission permission signal as its output to the subscriber unit (ONU), which in turn outputs the allowed amount of traffic.
FIG. 5 shows a mechanism for allocating a bandwidth in the next-generation PON (Passive Optical Network) system. In the current generation PON, such as G-PON (Gigabit capable PON) and GE-PON (Gigabit Ethernet-PON), in order to share a bandwidth, a packet multiplex technique, a kind of TDM (Time Division Multiplex) technique, is used, see FIG. 2. By contrast, the next-generation PON employs, in addition to the packet multiplex technique, a WDM (Wavelength Division Multiplex) or OCDM (Optical Code Division Multiplex) technique.
In the system shown in FIG. 5, for example, the subscriber units #0 to #(N−1) communicate over the channels ch0 to ch(N−1), respectively, as default channels. In FIG. 5, N is equal to 16. The ONUs functioning as part of subscriber units located in the respective subscriber premises and an OLT transport unit 503 functioning as part of the central-office equipment 501 report a bandwidth allowed for communication to an OLT controller 502, serving as a bandwidth calculating unit (step ST11, FIG. 5). This report includes information, such as a request for bandwidth and the queue length of a queue buffer in the subscriber unit. Thus, the signal in step ST11 may include a bandwidth request. Alternatively, in another method, either the ONU or OLT monitors the bandwidth being used to report information thereon. If the subscriber unit #1 requests communication in a bandwidth below that of the channel ch1 and another subscriber unit #0 requests communication in a bandwidth exceeding that of the channel ch0, the OLT controller 502 directs the subscriber unit #0 to communicate in an unused bandwidth within the channel ch1 in addition to the channel ch0 (step ST12, FIG. 5). That is, the signal in step ST12 may include a reply of transmission channel and time. This means that, while the subscriber unit #1 does not use the channel ch1, the subscriber unit #0 uses the channel ch1 for transmission. In this way, a free resource of bandwidth for each subscriber unit can be used by another subscriber unit, thereby increasing the usability of bandwidths. Incidentally, Japanese patent laid-open publication No. 153154/1993 discloses an exemplified system for managing and processing bandwidths of channels, wherein a bandwidth being used may be changed as needed.
In the above-described conventional techniques, many subscriber units can share bandwidths to increase the bandwidth usability. However, there are problems that the hardware implementing the sharing of bandwidths increases in volume and hence in cost. For example, the hardware implementing 16 subscriber units each of which can share bandwidths of 16 channels causes the cost of its transmitter/receiver to rise to 16-fold compared with the hardware without sharing bandwidths, that is, implementing 16 subscriber units each of which may use only the bandwidth of one channel.
In an exemplified method for sharing channels, each two of the subscriber units share two channels with each other, as shown in FIG. 6. In the figure, the subscriber units #0 and #1 share the channels ch0 and ch1, the subscriber units #2 and #3 share the channels ch2 and ch3, and the subscriber units #4 and #5 share the channels ch4 and ch5. However, a sharing method is expected that has higher bandwidth usability.